Transconductance elements are crucial and ubiquitous in many circuit blocks and microwave circuits are no exception. Active Gilbert type mixer requires a transconductance stage, also commonly referred to as voltage-to-current converter stage or in short V2I stage. Taking the example of a down-converting mixer, the V2I stage converts the RF input signal, usually a voltage signal to the current signal, which is supplied to the emitter or source terminals of the mixer switching core transistors.
The linearity (usually characterized as IIP3) and noise (conveniently computed as Noise Figure or NF) performance of the overall mixer are often set by the V2I stage itself, because of this being in the RF signal path. In designing a V2I stage, these are typically conflicting requirements and one must be traded off for the other. From the overall mixer performance point of view, higher IIP3 is desired when input signal is large and lower NF is desired when input signal is small, while lower IIP3 is acceptable under this condition. One of the common approaches to achieve this is putting a variable attenuator in front of the transconductance (V2I) stage. The V2I stage is then designed for the best desired NF with moderate IIP3. The variable attenuation in front of the V2I is adjusted to improve the IIP3 at the expense of NF. For every dB of attenuation inserted, the IIP3 improves by a dB and NF of the attenuator plus V2I combination becomes worse by a dB. This is sometimes referred to as dB for dB relationship.
This scheme works well at moderate RF frequencies, but at Microwave frequencies highly linear variable attenuator block is extremely challenging to implement. Some prior art references in this area are— US20120319755A1, U.S. Pat. No. 4,837,530A, U.S. Pat. No. 5,349,312A.
At present most state of the art high performance variable attenuators at microwave frequencies, are fabricated on InGaAs/GaAs processes using pHEMT and/or MEM-ET devices. But this creates a challenge in integration of these attenuators with other radio functions which are being implemented on advanced SiGe BiCMOS processes. So it would be desirable to implement a variable microwave attenuator (or gain) function on a modern SiGe BiCMOS process.